The present invention concerns electrical resistance elements and, in particular, compositions for making electrical resistance elements and methods of making the same.
Electrical resistance elements formed from certain compositions are particularly useful in producing microminiature circuitry for the electronics industry wherein electronic elements (or pastes) are screen printed onto substances.
U.S. Pat. No. 3,304,199 describes an electrical resistance element composed of a mixture of RuO.sub.2 or IrO.sub.2 and lead borosilicate glass. The mixture is combined with a vehicle, e.g., organic screening agent, such as ethyl cellulose dissolved in acetone-toluene. The resultant mixture containing the vehicle is applied onto a nonconductive substrate and then air fired.
U.S. Pat. No. 3,324,049 describes a cermet resistance material comprising 40 to 99 weight percent of a lead borosilicate glass, 0.5 to 20 weight percent of a noble metal such as Ag, Au, Pd, Pt, Rh, Ir, Os or Ru and 0.5 to 40 weight percent MnO.sub.2 or CuO. The resultant resistance material is then fired in air.
U.S. Pat. No. 3,655,440 concerns a resistance composition including RuO.sub.2, IrO.sub.2 or PdO, a lead borosilicate glass vitreous binder and an electrically nonconductive crystal growth controlling agent, e.g., alumina comprising submicron inert particles. Such resistance composition is air fired at 975.degree. C. to 1025.degree. C. for 45 minutes to 1 hour.
U.S. Pat. No. 3,682,840 concerns electrical resistor compositions containing lead ruthenate and mixtures thereof with RuO.sub.2, in conjunction with lead borosilicate binders.
U.S. Pat. No. 4,065,743 concerns a vitreous enamel resistor containing a glass frit and conductive particles. Such conductive particles include tin oxide and tantalum oxide.
U.S. Pat. No. 4,101,708 is directed to printable compositions of finely divided powder in an inert liquid vehicle for producing film resistors adherent to a dielectric substrate, such compositions including RuO.sub.2, glass containing PbO, Nb.sub.2 O.sub.5, CaF.sub.2 and an inert vehicle.
German Patentschrift 21 15 814 concerns a resistance paste for air firing on a ceramic. Such resistance paste includes BaRuO.sub.3, SrRuO.sub.3 and CaRuO.sub.3 in a lead borosilicate glass.
Resistor compositions have been made using Ag-Pd and/or PdO, RuO.sub.2, IrO.sub.2, and the so-called "du Pont" pyrochlores. The pyrochlore structures are complex oxides with the general formula A.sub.2 B.sub.2 O.sub.6-7 where the large cation A is in eightfold coordination and the smaller B cation is octahedrally coordinated. Their success is largely based on their stability in variable atmospheres (reducing) and their ability for handling multisubstitution of elements to alter electrical properties. Examples of pyrochlores specifically used in these compositions and discussed in U.S. Pat. Nos. 3,553,109; 3,560,410 and 3,583,931 (all of these patents involve lead borosilicate binders) include Bi.sub.2 Ru.sub.2 O.sub.7 and Pb.sub.2 Ru.sub.2 O.sub.7-x where O&lt;x&lt;1.
The resistivities of various precious metal oxides (including primarily pyrochlores and some perovskites) were tabulated by Bube, K., Proceedings of Inter. Microel. Symp., Oct. 30-Nov. 1, 1972, Washington, D.C., ISHM, as follows:
______________________________________ Oxide .rho.300.degree. K.'.sup..OMEGA.-cm ______________________________________ Rutile RuO.sub.2 3.5 .times. 10.sup.-5 IrO.sub.2 4.9 .times. 10.sup.-5 Rh.sub.2 O.sub.3 &lt;10.sup.-4 Pyrochlore Bi.sub.2 Ru.sub.2 O.sub.7 2.3 .times. 10.sup.-2 Bi.sub.2 Rh.sub.2 O.sub.6.8 3.2 .times. 20.sup.-3 Bi.sub.2 Ir.sub.2 O.sub.7 1.5 .times. 10.sup.-3 Pb.sub.2 Ru.sub.2 O.sub.6 2.0 .times. 10.sup.-2 Pb.sub.2 Ru.sub.2 O.sub.6.5 5.0 .times. 10.sup.-4 Pb.sub.2 Rh.sub.2 O.sub.7 6.0 .times. 10.sup.-1 Pb.sub.2 Ir.sub.2 O.sub.6.5 1.5 .times. 10.sup.-4 Pb.sub.2 Os.sub.2 O.sub.7 4.0 .times. 10.sup.-4 Tl.sub.2 Ru.sub.2 O.sub.7 1.5 .times. 10.sup.-2 Tl.sub.2 Ir.sub.2 O.sub.7 1.5 .times. 10.sup.-3 Tl.sub.2 Rh.sub.2 O.sub.7 6.0 .times. 10.sup.-4 Tl.sub.2 Os.sub.2 O.sub.7 1.8 .times. 10.sup.-4 Perovskite LaRuO.sub.3 4.5 .times. 10.sup.-3 La.sub..5 Sr.sub..5 RuO.sub.3 5.6 .times. 10.sup.-3 CaRuO.sub.3 3.7 .times. 10.sup.-3 SrRuO.sub.3 2.0 .times. 10.sup.-3 BaRuO.sub.3 1.8 .times. 10.sup.-2 ______________________________________
The perovskite crystal structure was described in Goldsmith, U. M., Skrifter Norske Videnskaps--Akad., Oslo, I: Mat. Nuturv.Kl. 2:8 (1926). In the perovskite composition of ABO.sub.3 the A cation is in twelve-fold coordination with oxygen and the smaller B cation is in octahedral coordination. This perovskite structure is one of high lattice energy and is generally a very stable structure.
Resistance compositions have been applied in screen printing techniques requiring firing in an oxidizing (air) atmosphere which necessitated the use of expensive noble metals such as Au, Ag, Pt and Pd. Less expensive copper as a base metal could not be employed since copper easily oxidizes. Accordingly, there is a need for a stable copper compatible resistance composition that could be fired in non-oxidizing atmospheres, e.g., nitrogen.
Typical previously employed resistance compositions utilized lead borosilicate glass binders. After firing in air, resistance compositions including, for example, strontium ruthenate in a lead borosilicate binder, the strontium would decompose to strontium oxide, which dissolves into the binder, and ruthenium oxide. In the present invention when, for example, strontium ruthenate in a strontium borosilicate binder is fired in nitrogen, there is no decomposition of the conductive component, i.e., the strontium ruthenate remains unchanged.